This invention relates to a drag reduction arrangement for the body of an athlete moving through a fluid medium.
Many athletic sports inherently involve the athlete moving his body through a fluid medium, usually air or water. Typically, such sports are swimming, cycling, skiing, and speed skating.
The drag on these athletes can be broken down into three main sub types, namely wave drag, skin friction drag, and form drag.
Wave drag results when a body moves on the surface of a fluid producing a wake (only the swimmer when on the surface of the water encounters this type of drag).
Skin friction drag results from the viscosity of the fluid and is applicable to all of the sports listed above. Fluid in contact with the surface of a body in motion decelerates to zero velocity with respect to the body. The difference in velocity this creates between the free stream flow and the skin of the body results in a finite shear region called a boundary layer. This boundary layer grows in thickness as it progresses from the front of the body to the back. In addition, the boundary layer may progress through two different states depending on flow conditions. In the laminar state, the flow is smooth and the skin friction drag is low. In the turbulent state, a laminar boundary layer may `transition` to a turbulent one under the right conditions. This results in higher skin friction.
Form drag occurs if boundary layer flow encounters an adverse pressure gradient, i.e., a region where the flow decelerates. The flow separates from the body resulting in the formation of large Eddies creating a low pressure region aft of the body. This can result in a dramatic increase in drag over a flow which remains attached. Unfortunately, laminar boundary layers, which have the lowest drag are also the most susceptible to separation.
With respect to the sports listed above, drag reduction efforts to date have largely concentrated on reducing skin friction drag. For example, it has been proposed to delay the development or reduce the intensity of a turbulent boundary layer by smoothing the body surface and employing streamwise riblets. Such a measure, however, has the effect of hastening boundary layer separation. A typical example of such a method is described in U.S. Pat. No. 5,033,116.
An alternative method involves covering the surface with a lubricant which is shed in the flow thereby reducing the net shear stress at the body surface.
However, since humans are not streamlined, surprisingly reducing the skin friction drag may actually increase the overall drag because delaying transition to a turbulent boundary layer or decreasing the intensity of the turbulent layer may induce earlier separation when the flow encounters an adverse pressure gradient, such as occurs in the vicinity of a curved surface. This effect may dramatically increase form drag, more than offsetting any gains from reduced skin friction.
In fact, a major source of drag in many racing sports results from flow separation at curved surfaces on the athlete's body, such as at the athlete's posterior, arms or back of the head. This is known as form drag. Specifically the sports which are most affected by this type of drag include any sport in which the athlete's torso is aligned with the direction of motion.
Classically, form drag is reduced by altering the shape of the object in the flow to more closely approximate a streamline shape. This is often not practical in the case of a human being. When practical, such methods are generally outlawed by the applicable sports governing body. For example, fairings are generally illegal in almost all sanctioned cycling races. Consequently, any method which reduces drag must also be subtle in order to avoid prohibition.
An object of the invention is to reduce the overall drag on an athlete moving through a fluid medium.